Better nutrition—leading to taller, heavier girls who mature younger—probably plays a role. Environmental exposure to hormone-mimicking chemicals may have an effect too. Pediatrician Barbara Cromer of Case Western Reserve University notes that many pesticides and plastics contain synthetic estrogens, and that cattle fattened with estrogen have up to five times as much of it in their tissue as do untreated cattle. “Early puberty could represent a ‘canary in the coal mine’ for excessive estrogen in our environment,” she says. If so, the next generation of young women are at greater risk of health problems. Elevated exposure to estrogen over a long period is linked with higher breast cancer rates in adulthood and earlier onset of risky sexual activity.